This invention is directed to negative-acting photoimageable compositions which are developable in alkaline aqueous solutions. The invention is particularly applicable to primary photoimaging resists, but is applicable, as well, to compositions that are hardenable so as to form solder masks and the like.
A variety of such photoimageable compositions are described. Essential components of compositions of the type to which the present invention is directed are A) a binder polymer; B) photopolymerizable .alpha.,.beta.-ethylenically unsaturated compound(s), and C) a photoinitiator chemical system. The binder polymer A) has sufficient acid functionality, generally carboxylic acid functionality, that it is soluble in alkaline aqueous solution and thereby renders the photoimageable composition developable in alkaline aqueous solutions.
Printed circuit boards generally have through-holes by which circuitry on one side of the board is connected to circuitry on the other side of the board. Photoresists are required to "tent" these through-holes during processing. Through-holes in printed circuit boards are increasingly becoming larger; accordingly, tenting strength is becoming increasingly important. It is a general object of the invention to provide photoimageable compositions with increased flexibility for increased tenting strength. Flexibility also contributes to cross hatch adhesion, allowing for better compatibility with polyester removal systems. This is because if the photoresist is brittle, polyester removal systems cause chipping of the photoresist and attendant circuit line defects.